The invention is based on a priority application EP03290493.0 which is hereby incorporated by reference.
The present invention relates to a Raman laser device having a first cavity in which lasing occurs at a first frequency, and at least one second cavity in which lasing occurs at a second frequency, thereby generating respective first and second waves inside the respective cavities having a first power and a second power, respectively, generating beams propagating outside the cavities by coupling out a part of the first power and a part of the second power, and attenuating that part of the second power that has been coupled out without attenuating the complementary part of the second power remaining in the second cavity.
Such a Raman laser device is per se known. Raman laser devices with more than one output wavelength are a promising means for second order pumping applications in optical telecommunication systems.
It is generally known in the field of optical telecommunication that optical signals propagating in a transmission line fiber may be amplified by a co-propagating or counterpropagating pump light wave by means of the Raman effect. Due to the Raman effect, pump light of a short wavelength undergoes a scattering interaction with the material of the fiber. Emitted light is shifted down in frequency. If the shifted down frequency matches the signal frequency, the signal may be amplified by stimulated Raman emission, in which signal light triggers the emission of scattered light.
The energy transfer may occur directly between a pump wave and the signal wave. Such a transfer is generally designated as first order pumping. Further, the first pump wave, that is the pump wave transferring energy to the signal, may itself be pumped by a second pump wave.
Such a pumping process including a second pump wave is called second order pumping.
In second order pumping applications, it is desirable to have a powerful second order pump wave pumping the fiber, which, along the fiber, transfers its power to the first order pump wave of longer wavelength. However, such an energy transfer premises that there is already a little power existing in the first order pump waves. Such a first order pump wave of little power is also called a seed. To generate such a seed, it is generally known to use laser diode sources in the 14xx nm range for launching the seeds in the fiber and to use a Raman laser emitting at a single powerful wavelength at about 1360 nm for pumping the seeds. However, to use laser diodes for launching the seeds requires multiplexing schemes that add cost to the device.
The power launched in the seeds should, on the one hand, be small in order to push forward the gain into the line fiber and consequently to improve the noise performance of the transmission. Further, the power in the seeds has to be above the lasing threshold in order to maintain a stable seed. Accordingly, it is desired to have a Raman laser device emitting at a powerful wavelength and at (at least) one seed wavelength simultaneously, the power emitted in the seed being stable and close to the lasing threshold.
The per se known Raman laser device mentioned at the outset emits simultaneously in the 1360 nm wavelength region and 14xx nm range. In order to have a stable power in the seeds in the 14xx nm range, the known laser works well above the lasing threshold (in the respective laser cavity/resonator) and the power of the seed wave that is coupled out of the laser is attenuated using a Long Period Grating (LPG). Such a Raman laser device is disclosed in the article “Dual-order Raman pump providing improved noise figure and large gain bandwidth”, Proceedings OFC 2002 (Anaheim, Calif.) Postdeadline paper FB3by J C Bouteiller et al.
However, a Long Period Grating is sensitive to temperature. Further, in order to have a spectral width that is small enough to select different output wavelengths in the 14xx nm range, a Long Period Grating requires about 50 mm of length, which is not compatible with current demands for compactness in the respective field of Raman laser devices. In addition, due to a distinct sensitivity to its external environment, a Long period Grating needs a specific packaging.